Here's a weird thing why is Mewtwo before mew in the pokedex well I have a few theory's
first is the show version- the pokedex is in order of each pokemon that was discovered by prof. oak in order ( but this wouldn't explain how the legendary pokemon are there)
the game version- The creators didn't want to confuse kids with a missing spot on the pokedex that couldn't be caught ( not counting events) so they made mewtwo first (but most kids were wondering why there was a mewtwo and not a mew one(or three) but that wouldn't explain why they've never changed it.